1. Field of the Invention
The present invention relates to an exhausting method of a vacuum tank in a device having a field emission type cold cathode, which can be used as an electron beam source for an electron microscope, an electron beam exposing device, a cathode ray tube (CRT), a flat panel display and other various electron beam devices.
2. Description of the Related Art
The field emission type cold cathode includes an emitter formed to be a cone-shaped sharp electron emission section and a gate layer having a sub-micron level radiation hole formed to be insulated against the emitter for exposing the same, each of which is disposed in a vacuum. This cold cathode serves as an electron source for emitting electrons from the tip part of the emitter into a vacuum when a positive voltage is applied to the gate layer against the emitter. For a manufacturing technique of such a field emission type cold cathode, reference may be made to, for example, the manufacturing method of a field emission type cold cathode using high melting point metal (molybdenum) for an emitter material presented on page 5248 of "Journal of Applied Physics. Vol. 47 (1976)".
A device which includes the field emission type cold cathode will be described below with reference to FIG. 1.
Emitters 1 are disposed on a conductive substrate 2 (alternatively, a conductive film formed on an insulated substrate). Gate layers 3 are disposed on an insulated layer 4 so as to surround the emitters 1, and a positive gate voltage 7 is applied against each emitter 1. An anode electrode 5 is positioned above the emitters 1, and a positive anode voltage 6 is applied against each emitter 1. Electrons are emitted from the tip part of the emitter 1 where an electronic field concentrates, and the emitted electrons flow into the anode electrode 5 having a positive voltage. A vacuum tank 8 is provided to isolate the emitters 1 and the anode electrode 5 from an atmosphere. The vacuum tank 8 is always exhausted by a vacuum pump 11 having a high exhaust speed, and preferably a very high vacuum state should be maintained. However, for a device which is not so large or heavy, typically, the vacuum tank 8 is completely detached from the exhaust system after vacuum exhaustion and then used under an isolated vacuum environment.
For example, for incorporating the field emission type cold cathode as an electron gun in a CRT, the exhausting process of the CRT goes as follows. Referring to FIG. 2, first, the neck section 12 of a CRT 14 and an exhaust line 15 are connected to each other by a connecting section 13, and the CRT 14 is exhausted by a vacuum pump 11 such as an oil diffusion pump or the like provided in the exhaust line. During exhaustion, the temperature of the CRT 14 must be maintained at 300.degree. C. to 400.degree. C. After exhaustion, the connecting section 13 between the neck section 12 of the CRT 14 and the exhaust line 15 is cut off and then a tip of the neck section 12 is sealed (tipped-off). Then, a getter 10 disposed in the CRT 14 is evaporated by high frequency induction heating performed from the outside and then stuck to the inner wall of the CRT 14. Since a getter 10 is chemically active, the getter 10 stuck to the inner wall of the CRT 14 absorbs residual gas inside the CRT 14 so as to further increase a vacuum level therein. Regarding the vacuum level inside the CRT 14 obtained by such an exhausting process, "Vacuum. Vol. 38" has reported on page 848 that a vacuum level is around 10.sup.-7 Torr and the major portion of residual gas is argon.
As described above, when the field emission type cold cathode is used in an independent vacuum tank such as a CRT, a high vacuum level of around 10.sup.-7 Torr is maintained. However, the effect of residual gas to an electron emission characteristic cannot be ignored in such a vacuum environment. In other words, as shown in FIG. 3, the residual gas has caused the deterioration of an electron emission characteristic after the passage of time, that is, image instability.
It has been known that the electron emission characteristic of the field emission type cold cathode is sensitive to the kind of residual gas in a vacuum for driving the same and the partial pressure of the residual gas. Particularly, the positive ions of the residual gas ionized by the emitted electrons are implanted to the emitters having negative potentials. Ion impacts then bring about an increase in current fluctuation and sputtering causes the permanent deformation or changes of the emitter tips. Consequently, great deterioration occurs in the electron emission characteristic and it is difficult to maintain a stable operation for a long time. Accordingly, in order to maintain a stable characteristic and increase the life of the device, a vacuum environment must be controlled by exhausting, to a permissible partial pressure, gas of a kind which damages the emitters.
In this regard, however, there are problems inherent in the conventional method. Specifically, control of residual gas performed during an exhausting process has been based on experience since gas of a kind which damages the emitters or its permissible partial pressure is not explicitly defined. As a result, deterioration with the passage of time has occurred in the electron emission characteristic and, once deteriorated, it has been impossible to restore the electron emission characteristic.